State of charge tracking system for battery systems based on relaxation voltage

ABSTRACT

A battery control module for a battery system comprises a voltage measuring module that measures battery voltage and a current measuring module that measures battery current. A state of charge (SOC) module that communicates with the current and voltage measuring modules and that estimates SOC based on relaxation voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/559,921, filed on Apr. 6, 2004. This application is related to U.S.patent application Ser. No. 11/081,979 filed on Mar. 16, 2005 and Ser.No. 11/081,978 filed on Mar. 16, 2005. The disclosures of the aboveapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to battery systems, and more particularlyto state of charge tracking systems for battery systems.

BACKGROUND OF THE INVENTION

Battery systems may be used to provide power in a wide varietyapplications. Exemplary transportation applications include hybridelectric vehicles (HEV), electric vehicles (EV), Heavy Duty Vehicles(HDV) and Vehicles with 42-volt electrical systems. Exemplary stationaryapplications include backup power for telecommunications systems,uninterruptible power supplies (UPS), and distributed power generationapplications.

Examples of the types of batteries that are used include nickel metalhydride (NiMH) batteries, lead-acid batteries and other types ofbatteries. A battery system may include a plurality of battery subpacksthat are connected in series and/or in parallel. The battery subpacksmay include a plurality of batteries that are connected in paralleland/or in series.

The maximum and/or minimum power that can be delivered by batteries,battery subpacks and/or battery systems varies over time as a functionof a temperature of the batteries, battery state of charge (SOC) and/orbattery age. Therefore, accurate estimation of battery SOC is importantto the determination of maximum and minimum power.

The energy that can be provided by or sourced to a battery is a functionof state of charge. When the battery state of charge is known andtargeted during operation, an optimal ratio can maintained between theability to accept amp-hours in charge and to provide amp-hours indischarge. As this optimal ratio can be maintained, there is a reducedneed to oversize the battery system to assure adequate power assist andregeneration energy.

For example in transportation applications such as HEVs or EVs, it isimportant for the powertrain control system to know the maximum and/orminimum power limit of the battery system. The powertrain control systemtypically receives an input request for power from an accelerator pedal.The powertrain control system interprets the request for power relativeto the maximum power limit of the battery system (when the batterysystem is powering the wheels). The minimum power limits may be relevantduring recharging and/or regenerative braking. Exceeding the maximumand/or minimum power limits may damage the batteries and/or the batterysystem and/or reduce the operational life of the batteries and/or thebattery system. Being able to estimate the battery SOC accurately hasbeen somewhat problematic—particularly when the battery system includesNiMH batteries.

SUMMARY OF THE INVENTION

A battery control module for a battery system comprises a voltagemeasuring module that measures battery voltage and a current measuringmodule that measures battery current. A state of charge (SOC) modulecommunicates with the current and voltage measuring modules andestimates SOC based on relaxation voltage.

In other features, the SOC module enables the SOC estimation when aqualified charge swing follows a discharge swing and relaxation. The SOCmodule enables the SOC estimation when a qualified discharge swingfollows a charge swing and relaxation. The SOC module accumulates chargeswing during charging and identifies the qualified charge swing when theaccumulated charge swing is within a charge swing window. The SOC moduleaccumulates discharge swing during discharging and identifies thequalified discharge swing when the accumulated discharge swing is withina discharge swing window.

In still other features, the SOC module monitors rest periods duringwhich the battery is neither charging nor discharging. The SOC moduleenables the SOC estimation when the rest period is greater than athreshold. The SOC module enables SOC estimation during charging when afirst period between the qualified charge swing and the prior dischargeswing and relaxation is less than a predetermined allowed time. The SOCmodule enables SOC estimation during charging when a second periodbetween the qualified discharge swing and the prior charge swing andrelaxation is less than a predetermined allowed time.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of a battery system includingbattery subpacks, battery control modules and a master control module;

FIG. 2 is a more detailed functional block diagram of a battery controlmodule;

FIG. 3 is an equivalent circuit of a battery;

FIG. 4 is a graph of battery current as a function of time;

FIGS. 5A and 5B are flowcharts illustrating steps of a relaxationvoltage approach for estimating state of charge;

FIG. 6 is a graph of battery current as a function of time with chargeand discharge swing and charge and discharge events shown; and

FIG. 7 is a flowchart illustrating a power ratio approach of estimatingbattery state of charge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, the same referencenumbers will be used in the drawings to identify the same elements. Asused herein, the term module or device refers to an application specificintegrated circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality. As used herein, theterm current swing refers to current integrated over a duration duringwhich the charge (polarity) is in one direction. Charge swing may beexpressed in units of Amp-seconds or A-s.

An exemplary system that can be used to calculate the SOC will be shown,although skilled artisans will appreciate that other systems may beused. Referring now to FIG. 1, an exemplary embodiment of a batterysystem 10 is shown to include M battery subpacks 12-1, 12-2, . . . , and12-M (collectively battery subpacks 12). The battery subpacks 12-1,12-2, . . . , and 12-M include N series connected batteries 20-11,20-12, . . . , and 20-NM (collectively batteries 20). Battery controlmodules 30-1, 30-2, . . . and 30-M (collectively battery control modules30) are associated with each of the battery subpacks 12-1, 12-2, . . .and 12-M, respectively. In some embodiments, M is equal to 2 or 3,although additional or fewer subpacks may be used. In some embodiments,N is equal to 12-24, although additional and/or fewer batteries may beused.

The battery control modules 30 sense voltage across and current providedby the battery subpacks 12. Alternatively, the battery control modules30 may monitor one or more individual batteries 20 in the batterysubpacks 12 and appropriate scaling and/or adjustment is performed. Thebattery control modules 30 communicate with a master control module 40using wireless and/or wired connections. The master control module 40receives the power limits from the battery control modules 30 andgenerates a collective power limit. The SOC can be calculated for eachmodule, in groups and/or collectively. The battery control module 30 maybe integrated with the master control module 40 in some embodiments.

Referring now to FIG. 2, some of the elements of the battery controlmodules 30 are shown. The battery control modules 30 include a voltageand/or current measuring module 60 that measures voltage across thebattery subpack 12 and/or across one or more individual batteries 20 inthe battery subpack 12. The battery control modules 30 further include abattery state of charge (SOC) module 68 that periodically calculates theSOC of the batteries 20 in the battery subpacks 12. In oneimplementation, the SOC module 68 uses a power ratio estimation and/orV₀ approach, as will be described below. In another implementation, theSOC module 68 uses a relaxation voltage SOC estimation approach, as willbe described below. The SOC module 68 may employ a lookup table 70,formulas and/or other methods.

A power limit module 72 calculates a maximum current limit I_(lim),voltage limit V_(lim), and/or power limit P_(lim) for the batterysubpack 12 and/or one or more batteries 20 in the battery subpack 12, aswill be described further below. The limits may be maximum and/orminimum limits. A contactor control module 74 controls one or morecontactors (not shown) that are associated with the control and/orconnection of the batteries 20 in the battery subpacks 12. A clockcircuit 76 generates one or more clock signals for one or more moduleswithin the battery control module 30.

Referring now to FIG. 3, an equivalent circuit for the battery 20 isshown where R₀ represents ohmic resistance of the battery, V_(p)represents the polarization voltage, V₀ represents the open circuit orrelaxation voltage, I represents battery current and V representsbattery voltage. V and I are measured values. R_(p) varies withtemperature, duration of applied current and SOC. V₀ and R₀ varyprimarily with SOC. V_(p) is equal to measured current I times R_(p).Using the equivalent circuit and Kirchoff's voltage rules for thebattery 20, V=V₀+V_(p)+IR₀.

Relaxation voltage is relatively insensitive to temperature and currentdemand and is a good indicator of SOC. A set of specialized currentpulses can be used to condition the battery to yield SOC dependentrelaxation voltages. This approach is referred to herein as relaxationvoltage SOC estimation.

Referring now to FIG. 4, battery current is shown as a function of time.Current that is greater than zero, for example at 100-1, 100-2, 100-3,and 100-4, is charging current. Current that is less than zero, forexample at 102-1, 102-2, and 102-3, is discharging current. The areasunder the curve between points 106 and 108 and points 110 and 112 aredefined as a charge swing in A-s. The area under the current curvebetween points 108 and 110 is defined as a discharge swing in A-s.

Referring now to FIGS. 5A and 5B, steps of a method for implementing arelaxation voltage SOC estimation approach are shown. The relaxationvoltage estimation approach monitors battery current for a pair of powerpulses, checks relaxation voltage after each and determines SOC usingthe lookup table 70. The relaxation voltage approach was derived basedon the observation of voltage responses to pulses throughout a rangeoperating of temperatures, such as −15° C. to 45° C. The relaxationvoltages were affected by swing amplitudes, pulse amplitudes and whetherthe battery was brought from top of charge or bottom of charge.

In FIGS. 5A and 5B, control begins with step 150. In step 152, thecurrent and voltage are measured. In step 154, control determineswhether the measured current is charge current (current>zero or apredetermined threshold). If step 154 is true, control accumulatescharge swing and resets discharge swing in step 156. In step 158,control sets a rest variable equal to zero. In step 162, controldetermines whether the accumulated charge swing is within apredetermined window. The window may include upper and lower thresholds.In some implementations, the upper and lower thresholds are between 10%and 100% of battery capacity, although other values may be used. If not,control disables SOC lookup after charge in step 163 and returns to step152.

If step 162 is true, control continues with step 164 and determineswhether last swing and relaxation occurred in discharge. As used herein,relaxation refers to battery voltage asymptotically approaching therelaxation voltage. If not, control continues with step 163. If step 164is true, control enables SOC lookup after charge in step 166.

If step 154 is false, control continues with step 174. In step 174,control determines whether the measured current is discharge current(current<zero or a predetermined threshold). If step 174 is true,control accumulates discharge swing and resets charge swing in step 176.In step 178, control sets the rest variable equal to zero. In step 182,control determines whether the accumulated discharge swing is within apredetermined window. The window may include upper and lower thresholdsthat may be similar to the accumulated charge swing thresholds ordifferent therefrom. If not, control disables SOC lookup after dischargein step 183 and returns to step 152.

If step 182 is true, control continues with step 184 and determineswhether last swing and relaxation occurred in charge. If not, controlcontinues with step 183. If step 184 is true, control enables SOC lookupafter discharge in step 186.

If step 174 is false, control continues in FIG. 5B with step 200 andincrements the rest variable. In step 202, control determines whetherrest time is adequate by comparing rest time to a threshold. In someimplementations, approximately 120 seconds is used as a threshold,although other values may be used. If step 202 is true, controldetermines whether allowable time is less than a threshold timeTh_(time) in step 204. In some implementations, allowable time is equalto 240 seconds, although other values may be used. Exceeding this valuetends to indicate that the pulses were not controlled enough for an SOCestimation.

If step 204 is true, control continues with step 206 and determineswhether SOC lookup after charge is enabled. If step 206 is true, controllooks up SOC as a function of relaxation voltage in step 208 anddisables SOC lookup after charge in step 210 and control returns to step152. If step 206 is false, control continues with step 212 anddetermines whether SOC lookup after discharge is enabled. If step 212 istrue, control looks up SOC as a function of relaxation voltage in step214 and disables SOC lookup after discharge in step 216 and controlreturns to step 152. If steps 202, 204 or 212 are false, control returnsto step 152.

The power ratio SOC estimation approach monitors power pulse pairs. Themethod calculates the ratio of power capabilities in charge anddischarge when the swings of the pulse pairs are approximately equal.The SOC is a function of the power ratio and is determined by a lookuptable. The algorithm was derived while attempting to use inputs ofcurrent and voltage to solve for relaxation voltage V₀.

The voltage equation as the maximum or minimum power is held to avoltage limit is V_(lim)=V₀+V_(p)+I_(lim)R₀. Substitution of thecalculation for V₀+V_(p) from a prior sampling interval into theequation for V_(lim) yields V_(lim)=(V−IR₀)+I_(lim)R₀. In this case, weare assuming that V₀+V_(p) for the current sampling interval isapproximately equal to V₀+V_(p) of the prior sampling interval (in otherwords, V₀+V_(p)≅V_(t=i−1)−I_(t=i−1)R₀). This approximation is valid ifthe sampling interval is sufficiently small since the battery andambient conditions are very similar. For example in someimplementations, a sampling interval 10 ms<T<500 ms may be used,although other sampling intervals may be used. In one embodiment, T=100ms. Sampling intervals of 1 second have been used successfully. If thesampling interval is determined to be excessive in duration then R₀would be increased as a constant or as a temperature dependent variable.

Solving for I_(lim) yields the following:

$I_{\lim} = {\frac{V_{\lim} - V_{t = {i - 1}} + {I_{t = {i - 1}}R_{0}}}{R_{0}}.}$Therefore, since P_(lim)=V_(lim)I_(lim),

$P_{\lim} = {{V_{\lim}( \frac{V_{\lim} - V_{t = {i - 1}} + {I_{t = {i - 1}}R_{0}}}{R_{0}} )}.}$

At the time that power limit is established for a charge or dischargeswing and measured current, the measured current and voltage values arestored. When the current is reversed, the swing amplitude passes thenegative of the retained swing, and the current is approximately equalto the magnitude of the retained current, a power limit calculation isperformed.

The power ratio is calculated by taking P_(lim) in charge divided by−P_(lim) in discharge for adjacent cycles. Even though V₀ and V_(p) areno longer in the equation, their contributions are reflected in currentand voltage measurements, which are functions of both the polarizationbuild up and V₀. The polarization voltage V_(p) during a charge swing isapproximately equal to the polarization voltage V_(p) during a dischargeswing of approximately equal magnitude. Using this approximation, thepower ratio SOC estimation is used to remove V_(p) from the calculation.The use of the power limit ratio has the effect of adding considerationof the low discharge power at low SOC and the low charge acceptance athigh SOC to the stated charge determination.

In FIG. 6, the battery current is shown. The present invention monitorscharge and discharge swing and declares charge and discharge eventsunder certain circumstances. A charge swing event occurs when the chargeswing is greater than a charge swing threshold. A discharge event occurswhen a discharge swing is greater than a discharge swing threshold. Thethresholds may be related to or based on a prior charge or dischargeevent. For example, a charge swing threshold may be set equal to theabsolute value of a prior discharge event. A discharge swing thresholdmay be set equal to the absolute value of a prior charge event. Stillother approaches may be used to determine the charge and dischargethresholds. As used herein, the term claim refers to situations when acharge or discharge event is followed by a discharge or charge claim andwhen other conditions described below are met. The occurrence ofdischarge event is determined independently from the occurrence of thedischarge claim, to different criteria. The algorithm looks for bothsimultaneously. For example, the claim point occurs at the time that thearea discharge swing is equal to the previous charge swing. The eventpoint occurs when the ratio current vs. discharge current MIN is roughlyequal to the ratio current at charge event vs. charge current MAX. Thiswould be the case if L=K in FIG. 7. In some implementations, L and K arebetween 1 and 2, although other values may be used.

Referring now to FIG. 7, the power ratio SOC estimation method accordingto the present invention is shown in further detail. Control begins withstep 250. In step 254, control measures current and voltage. In step258, control determines whether there is a charge current. Chargecurrent is defined by positive current above zero or a predeterminedpositive threshold. If step 258 is true, control continues with step 262and accumulates charge swing. In step 264, control determines whetherthe current during the charge swing passes a maximum value and isgreater than Current_(max)/K. When step 264 is true, control storesvalues of current, charge swing and power limit in step 266. If not,control continues past step 266 to step 270. In step 270, controldetermines whether the swing is greater than the prior discharge swing.If not, control does not make an SOC claim in step 272 and controlcontinues with step 254.

If step 270 is true, control determines whether the current isapproximately equal to a retained discharge current −I_(DR) (in otherwords within upper and lower thresholds thereof) in step 274. If step274 is false, control does not make an SOC claim in step 276 and controlcontinues with step 254. If step 274 is true, control looks up SOC as aratio of power limit to retained power limit in step 280.

If step 258 is false, control continues with step 278 and determineswhether discharge current is present. Discharge current is present whendischarge current is less than zero or a predetermined negativethreshold. If step 278 is false, control returns to step 254. If step278 is true, control continues with step 282 and accumulates dischargeswing. In step 284, control determines whether the current during thedischarge swing passes a minimum value and is less than Current_(min)/L.When step 284 is true, control stores values of current, the dischargeswing and power limit in step 286. If not, control continues past step286 to step 290. In step 290, control determines whether the dischargeswing is greater than the prior charge swing. If not, control does notmake an SOC claim in step 292 and control continues with step 254.

If step 290 is true, control determines whether the current isapproximately equal to a retained charge current −I_(CR) (in other wordswithin upper and lower thresholds thereof) in step 294. If step 294 isfalse, control does not make an SOC claim in step 296 and controlcontinues with step 294. If step 294 is true, control looks up SOC as aratio of power limit to retained power limit in step 300.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

1. A battery control module for a battery system, comprising: a voltagemeasuring module that measures battery voltage; a current measuringmodule that measures battery current; and a state of charge (SOC) modulethat communicates with said current and voltage measuring modules andthat estimates SOC based on relaxation voltage, wherein said SOC moduleenables said estimation of said SOC based on one of a qualified chargeswing following a discharge swing and a qualified discharge swingfollowing a charge swing.
 2. The battery control module of claim 1wherein said SOC module enables said estimation of said SOC when saidqualified charge swing follows said discharge swing and relaxation. 3.The battery control module of claim 1 wherein said SOC module enablessaid estimation of said SOC when said qualified discharge swing followssaid charge swing and relaxation.
 4. The battery control module of claim2 wherein said SOC module accumulates charge swing during charging andidentifies said qualified charge swing when said accumulated chargeswing is within a charge swing window.
 5. The battery control module ofclaim 3 wherein said SOC module accumulates discharge swing duringdischarging and identifies said qualified discharge swing when saidaccumulated discharge swing is within a discharge swing window.
 6. Thebattery control module of claim 1 wherein said SOC module monitors restperiods during which said battery is neither charging nor discharging.7. The battery control module of claim 6 wherein said SOC module enablessaid estimation of said SOC when said rest period is greater than athreshold.
 8. The battery control module of claim 2 wherein said SOCmodule enables SOC estimation during charging when a first periodbetween said qualified charge swing and said prior discharge swing andrelaxation is less than a predetermined allowed time.
 9. The batterycontrol module of claim 3 wherein said SOC module enables SOC estimationduring charging when a second period between said qualified dischargeswing and said prior charge swing and relaxation is less than apredetermined allowed time.
 10. A method for operating a battery controlmodule for a battery system, comprising: measuring battery voltage;measuring battery current; estimating state of charge (SOC) based onrelaxation voltage, said battery voltage and said battery; and enablingsaid SOC estimation when one of a qualified charge swing follows adischarge swing and a qualified discharge swing follows a charge swing.11. The method of claim 10 further comprising enabling said SOCestimation when said qualified charge swing follows said discharge swingand relaxation.
 12. The method of claim 10 further comprising enablingsaid SOC estimation when said qualified discharge swing follows saidcharge swing and relaxation.
 13. The method of claim 11 furthercomprising: accumulating charge swing during charging; and identifyingsaid qualified charge swing when said accumulated charge swing is withina charge swing window.
 14. The method of claim 12 further comprising:accumulating discharge swing during discharging; and identifying saidqualified discharge swing when said accumulated discharge swing iswithin a discharge swing window.
 15. The method of claim 10 furthercomprising monitoring rest periods during which said battery is neithercharging nor discharging.
 16. The method of claim 15 further comprisingenabling said SOC estimation when said rest period is greater than athreshold.
 17. The method of claim 11 further comprising enabling SOCestimation during charging when a first period between said qualifiedcharge swing and said discharge swing and relaxation is less than apredetermined allowed time.
 18. The method of claim 12 furthercomprising enabling SOC estimation during charging when a second periodbetween said qualified discharge swing and said charge swing andrelaxation is less than a predetermined allowed time.